In wireless systems, the transmitted signal may undergo severe, distortion due to signal dispersion which can be frequency selective. Signal dispersion can occur, for example, when a transmitted wireless signal bounces off the walls in which a cell phone user is located. The received signal will then include signal components from the multiple paths of transmission.
Equalization is a technique generally used to combat frequency-selective fading, a type of distortion that manifests itself as inter-symbol interference (ISI). The distortion caused by ISI falls into two different categories: inter-codeword interference and intra-codeword interference. Inter-codeword interference is the distortion caused on a transmitted codeword due to the presence of other codewords in the transmission stream. Intra-codeword interference is the distortion caused on a transmitted codeword even when no codewords are transmitted prior to or after the codeword. ISI can further be categorized as pre-cursor, cursor and post-cursor ISI which relate to the differing paths of a received wireless signal.
To remove ISI, various equalization techniques are known that have differing complexities and performance. One type of equalizer is decision-feedback equalizer (DFE), which has the appeal of low complexity and good performance. The DFE typically includes a feed-forward filter that suppresses precursor ISI and a feedback filter that suppresses post-cursor ISI. One disadvantage of a DFE is that it requires delay-free decision feedback necessary to operate the DFE. The input to the feedback filter requires a delay-free decision on individual input symbols of the decision device and is thus not very reliable.
U.S. Pat. No. 6,233,273 discloses an equalizer that attempts to eliminate the requirement of delay-free feedback at the input to the feedback filter of the DFE. One approach disclosed in the '273 patent is to use multiple feedback paths upstream of the correlator: one feedback path corresponding to each possible codeword, in which each of the codewords is input into its own feedback path. To determine the actual transmitted codeword, a correlation is performed on each of the multiple DFE outputs with respect to the corresponding codeword and the output that has the highest correlation is chosen. The equalizer of the '273 patent, however, has considerable complexity due to its multiple feedback paths and corresponding multipath correlations that need to be performed to determine the actually transmitted codeword.
Another approach of the '273 patent is to use multiple feedback paths downstream of a codeword correlator. A Fast Walsh Transform system is implemented as a codeword correlator for the received signal. Typically, however, the output signals of the Fast Walsh Transform system include an undesired bias when only partial equalization has been performed upstream of the Fast Walsh Transform. The '273 patent discloses an attempt to remove this bias by sending the outputs of the Fast Walsh Transform system to an expander which expands the number of outputs such that each of the outputs can have subtracted from it one of a plurality of correlated signals, one for each possible codeword. A peak detector then determines which of these outputs is the actual transmitted signal. As with the upstream correlation method described above, this method also has considerable complexity due to the multiple feedback paths and the corresponding multipath correlation that needs to be performed for each of the possible codewords.